The invention relates to a lift apparatus for lifting heavy loads, particularly a lift platform, comprising a plurality of cylinder-piston assemblies, arranged in pairs, comprising one hydraulic medium supply and one hydraulic medium drain each, in which each pair of assemblies comprises a cylinder-piston assembly connected to a pressure connection of at least one hydraulic pump, operating as a master assembly, and the respectively other cylinder-piston assembly is operated as a slave assembly, in which its hydraulic medium supply is connected to the hydraulic medium drain of the cylinder-piston assembly operated as a master assembly.
A generic lifting apparatus in the form of a scissor platform is known from the publication DE 29916254U1. It comprises two scissor frames, which are operated from a pair of assemblies. One cylinder-piston assembly of each pair of assemblies serves as a master assembly and is hydraulically coupled to the second cylinder-piston assembly of the other pair of assemblies, acting as a slave assembly. This is advantageous in that two separate circuits are provided, so that in case of any leak or a break of a hydraulic line the lift platform is prevented from lowering, because the assembly of the second hydraulic circuit, not affected by the defect, takes of the holding function of the defective assembly for both scissor frames.
Different design principles are known for lift platforms. In addition to scissor platforms, primarily columns or piston lift platforms are used. Usually they are operated hydraulically. When a transfer of force occurs during the lifting via more than one point of action, thus over several lift elements, the strokes of the hydraulic drives involved must be synchronized in order to ensure a homogenous and simultaneous lifting process.
In case of two points of action, a command/response and/or master/slave-arrangement can be used. Here, dual-action hydraulic cylinders with a hydraulic medium supply and a hydraulic medium drain are used. A first hydraulic cylinder serves as the master cylinder, with its hydraulic medium drain being connected to the hydraulic medium supply of the second hydraulic cylinder, operating as the response cylinder. Thus, the master cylinder and the response cylinder form a closed hydraulic circuit so that their strokes are mandatorily synchronized.
In the event a lifting process needs to occur simultaneously at more than two hosting points, synchronization of the hydraulic cylinders required for this purpose occurs usually by a respective active control circuit by measuring the strokes or the displaced volume of hydraulic fluid. This is expensive and subject to errors, though.
Additionally, in hydraulic lift platforms it is important for safety reasons that in case of a hydraulic failure a raised vehicle cannot descend unintentionally and in an uncontrolled fashion. Accordingly, frequently additional locking latches, brakes, or other fastening elements are used, which in case of a defect prevent any descending.